1. Field of the Invention
This invention relates generally to the recovery of liquid and gaseous products from carbonaceous materials such as coal, char, tar sands, oil shale, uintaite and biomass and more particularly concerns the rapid and direct conversion of such carbonaceous materials involving hydropyrolysis in the gas phase.
2. Description of the Prior Art
Considerable evidence in the literature suggests that the products initially formed during the thermal decomposition of carbonaceous materials such as coal, char, tar sands, oil shale, uintaite and biomass are largely in the liquid molecular weight range and that they continue to decompose and recombine to form refractory products like coke and gas the longer they are subjected to the thermal decomposition conditions. The available evidence also indicates that the lifetime of these liquid products, under the conditions of thermal decomposition is short. Therefore, in order to maximize liquid yields from such decompositions, it is desirable to limit the time during which the products initially formed are subjected to the decomposition conditions. Thus a low residence time of the decomposition mixture in the decomposition zone and a high rate of decomposition therein are advantageous. Similarly, rapidly quenching the decomposition reaction at some optimum short time after the decomposition commences reduces undesirable secondary reactions.
Decomposition at low pressures also maximizes the yield of the desired hydrocarbon liquids and gases. The use of low decomposition pressures facilitates the escape of volatile products from the decomposing carbonaceous material and from one another and thus minimizes their tendency to recombine.
Furthermore, it is generally recognized that the conversion of carbonaceous materials, such as coal, char, tar sands, oil shale, uintaite and biomass, to the desired liquid and gaseous products can be maximized by stabilizing the liquid products initially formed. This is often effected by reaction of the liquid products with a stabilizing material such as hydrogen or with a source of such stabilizing material. It has been shown that at the beginning of the thermal decompositions of such carbonaceous materials a transient period exists during which the products initially formed are highly reactive toward a stabilizing material such as hydrogen. The overall effect of thermal decomposition in the presence of such a stabilizing material or a source thereof is a much larger yield of the desired liquids and a lower char yield. However, if excess stabilizing material is not readily available during this period, some of the free radical decomposition products will polymerize to form unreactive char, with the overall effect being a limited yield of the desired liquids and a large yield of char.
However, decomposition implies that chemical bonds are being broken inside the carbonaceous material where the products initially formed may be effectively insulated from the stabilizing material or a source thereof in the environment surrounding the carbonaceous material and thus are precluded from stabilization by reaction with the stabilizing material or a source thereof. Moreover, the available time to achieve such stabilization may be too short to rely on mass transfer of the stabilizing material or a source thereof solely by diffusion and convection. Decomposition at low pressures facilitates the escape of volatile products from the decomposing carbonaceous material and from each other and thereby enhances their accessibility to the surrounding environment of stabilizing material or a source thereof. Moreover, pretreatment to position the stabilizing material or source thereof in extremely close proximity to the carbonaceous material before decomposition commences minimizes the effects of such slow mass transfer.
Greene, U.S. Pat. Nos. 3,997,423; 4,012,311; 4,013,543; and 4,048,053; Rosen et al., 3,960,700; and Pelofsky et al., 4,003,820 disclose processes for recovering liquids from carbonaceous solids and lower boiling liquids from higher boiling liquid hydrocarbons, which do involve a rapid decomposition of the carbonaceous material in the presence of hydrogen and at a low pressure and a rapid quenching of the decomposition reaction.
In particular, Greene, U.S. Pat. Nos. 3,997,423 and 4,013,543 disclose a process of producing carbonaceous tars from liquid or crushed solid carbonaceous material comprising (1) introducing carbonaceous material into a reactor; (2) adding hot hydrogen to the carbonaceous material in the reactor; (3) reacting the hydrogen and carbonaceous material for a period of from about two milliseconds to about two seconds at a temperature of about 400.degree. C. to 2,000.degree. C. and at a pressure between atmospheric and 250 psia.; and (4) quenching the mixture within the reactor, with the total residence time for steps (2) and (3) varying from about two milliseconds to about two seconds. The patentee states that the heat-up rate of the carbonaceous material is in excess of 500.degree. C. per second.
Greene, U.S. Pat. Nos. 4,012,311 and 4,048,053 discloses processes which are similar to the processes of Greene, U.S. Pat. Nos. 3,997,423 and 4,013,543, and in which the decomposition reaction takes place at a pressure between atmospheric and 450 psia.
Rosen et al., U.S. Pat. Nos. 3,960,700 and Pelofsky et al., 4,003,820 disclose processes which are similar to the processes of Greene, U.S. Pat. Nos. 3,997,423 and 4,013,543, and in which the decomposition reaction takes place at a higher pressure between 500 and 5,000 psig.
Although Pelofsky et al., U.S. Pat. No. 4,003,820 and Greene, U.S. Pat. Nos. 4,012,311 and 4,048,053 do disclose in general terms an additional step in which the carbonaceous material is pretreated with hydrogen prior to being decomposed, such patents do not disclose the conditions of such pretreatment.
Furthermore, none of Greene, U.S. Pat. Nos. 3,997,423; 4,012,311; 4,013,543; and 4,048,053; Rosen et al., U.S. Pat. No. 3,960,700; or Pelofsky et al., U.S. Pat. No. 4,003,820 disclose a suitable method for rapidly introducing the carbonaceous material into the reactor. These patents disclose only that, in order to overcome the reactor pressure, both the carbonaceous material and the incoming hydrogen must be fed into the reactor at a pressure exceeding that of the reactor. Rapid passage of the carbonaceous material into and through the reactor is essential if a short decomposition time and a commercially acceptable, high through-put of carbonaceous material is to be achieved.
One suitable method for rapidly introducing the carbonaceous material into the decomposition zone involves entraining the carbonaceous material in a stream of compressed gas and instantaneously expanding and accelerating this stream as it passes through a restricted area into the decomposition zone. A similar technique is employed in a method for disintegrating coal solids as disclosed in Yellott, U.S. Pat. No. 2,515,542. Such technique not only serves to introduce the carbonaceous material rapidly into the decomposition zone but also permits the volatile fragments and radicals which form in the interior of the carbonaceous material to move rapidly away from the carbonaceous material and from one another.
Avco Everett Research Laboratory, Inc. has in very general terms disclosed to various people in the industry a coal gasification technique utilizing a two-stage gasifier. In the first stage, char is burned with oxygen to generate heat. The combustion gases from this combustion are then fed to a pyrolyzer through a converging-diverging nozzle. A large pressure drop is maintained across the nozzle. The combustion gases are accelerated to sonic conditions in the converging section of the nozzle, resulting in a cooling of the gases. Coal and steam are fed or aspirated into the stream of combustion gases at or slightly upstream of the throat of the nozzle. The mixture is then accelerated to supersonic flow in the diverging section of the nozzle and discharges into the pyrolyzer as a confined jet. As the gas velocity decreases from supersonic flow to subsonic flow in the pyrolyzer, a shock occurs which results in rapid heating of the coal, leading to the rapid formation of volatile material in the coal. Many of the volatiles are believed to be free radicals which are stabilized by the steam, thus preventing soot formation. Argon, carbon monoxide, helium and nitrogen have also been studied as stabilization gases. The residence time of the reaction mixture in the pyrolyzer is about 40 milliseconds.